Automatic precipitator voltage control



Mayv 13, 1969 J. W. DRENNING AUTOMATIC PRECIPITATOR VOLTAGE CONTROL Filed Ju e 11, 1965 Sheet of-2 INVENTOR JOHN W. DRENNING L LT:

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ATTORNEYS 1969 J. w. DRENNING 3,443,361

AUTOMATIC PRECIPITATOR VOLTAGE CONTROL Filed June 11, 1965 Sheet 2 of 2 I SAMPLING PERIOD A 4637; 47

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TIME

I nnn 1 nnn H64 69 MOTOR 2o RUNS (DECREASE) MOTOR 2o RUNS I (INCREASE) i=0 r=0 1 no NO SPARK :SPARK 0N HIGH {SPARK 0N HIGH SIGNAL ONLY SIGNAL SPARK 0N LOW SIGNAL I A JOHN W. DRENNING TNVENTOR 5 p 1 ATTORNEYS United States Patent 3,443,361 AUTOMATIC PRECIPITATOR VOLTAGE CONTROL John W. Drenuing, Baltimore, Md., assignor to Koppers Company, llnc., Pittsburgh, Pa., a corporation of Delaware Filed June 11, 1965, Ser. No. 463,126 llnt. Cl. B03c 3/68 US. Cl. 55-105 13 Claims ABSTRACT OF THE DISCLOSURE A system for maintaining precipitator voltage at a predetermined operating value below sparkover potential by superimposing one, or a pair, of unidirectional or high frequency sampling waveforms of predetermined amplitude recurrently on the precipitator electrodes and adjusting the potential applied to the electrodes in response to precipitator sparking during the application of the sampling waveforms.

The present invention relates to a system for maintaining the energizing voltage applied to an electrostatic precipitator at a desired operating value, below the instantaneous sparkover potential of the precipitator. In the dynamic environment in which most electrostatic precipitators operate, the sparkover voltage continually varies, presenting a long recognized problem of maintaining the operating voltage at a preferred value slightly below the sparkover potential. The prior art has approached this problem on a statistical basis, in some instances by varying the operating voltage in dependency on spark rate to maintain the sparkover frequency at some preselected value, such as one hundred cycles a minute. In other known systems, in response to a sparkover, the operating potential is susbtantially lowered and then automatically raised until a subsequent sparkover is encountered.

According to the present invention, the operating potential of the precipitator is maintained at a substantially constant preselected value below that at which sparkover occurs. This is accomplished by periodically applying an incremental voltage to the operating voltage, which samples the instantaneous operating characteristics of the precipitator in the voltage region above the operating voltage. If during the application of the incremental voltage, the precipitator does not sparkover, the operating voltage is automatically increased until sparkover is encountered during application of the sampling voltage, at which value the operating voltage is stabilized. The system of the present invention permits a sparkover rate which may be materially lower than that obtained in prior systems, since the ecurrence rate at which the sampling pulses are applied maybe selected as desired.

The present invention, moreover, provides for limiting the duration of sparkover by employing a sampling pulse waveform which accelerates deionization and does not increase the average potential developed across the precipitator electrodes during its application. It is generally recognized that the energy discharge effectuated during sparkover of a precipitator represents useless dissipation,

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for it serves no function in the system. Consequently, it represents an important advance in the art to permit the minimization in frequency of such occurrences, and perhaps an even more important advance in the art to minimize the energy loss in each sparkover by limiting its I duration to decrease the charge loss and accelerate recovery of the operating potential to a value in the efiicient precipitating range. Limitation of energy discharged during a sparkover, moreover, serves to avoid or to limit current surges in the power supply network which can otherwise become extremely objectionable from an engineering viewpoint.

It is a further feature of the present invention to employ, in connection with the sampling pulses above described, an intervening series of incremental voltages of lower value applied pulsewise to increase the operating potential momentarily. If no sparkover occurs during the application of such a low level pulse, the precipitator supply potential is maintained constant. If sparkover does occur, operating potential is decreased. Thus, by selecting desired values for the alternate incrementally applied voltages, the operating voltage of the precipitator will be maintained at a value below sparkover voltage, which voltage as it instantaneously varies, will lie between the total voltage on a precipitator during the lower valued sampling pulse and the total precipitator voltage developed during the application of the higher valued sampling pulse. Therefore, as the instantaneous value of the sparkover voltage changes due to the entrained gaseous material passing through the precipitator, the operating voltage is properly modulated to conform to a desired value below sparkover potential. The invention is particularly applicable to direct current energized precipitators, such as those fed by full wave rectification of cycle power. The pulse waveform which results in minimizing energy loss during sparkover, however, may advantageously be employed in pulse-energized precipitator systems wherein the electrodes are periodically charged to a predetermined potential which gradually subsides to lesser values during the power pulse intervals.

Thus, it is a primary object of the invention to maintain constant control of the operating voltage of an electrostatic precipitator.

It is a further object of the present invention to maintain the energizing voltage of the precipitator consistently near the sparkover level, at a point of preferred operating efiiciency.

A further object of the invention is to provide a sampling pulse control in a precipitator using conventional direct current power supply means.

Another object of the present invention is the use of sampling pulses of high frequency voltage incrementally applied across the precipitator electrodes momentarily to raise the operating voltage.

A preferred specific embodiment of the invention, as well as other objects and advantages thereof, will appear more clearly in the following detailed description in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic circuit embodying the principles of the automatic precipitator control system,

FIGURE 2 diagrammatically shows voltage values in a system employing a periodic sampling pulse superimposed on the operating precipitator voltage,

FIGURE 3 diagrammatically shows voltage values where two alternate series of voltage pulses are incrementally applied at different amplitudes,

FIGURE 4 diagramatically shows voltage values in the application of pulses of high frequency energy incrementally to the operating potential of a precipitator, and

FIGURE 5 shows time sequences of operation under different conditions encountered for the circuit of FIG. 1.

The system of FIG. 1 for operating an electrostatic precipitator schematically shows a pair of precipitator elements and 11. The precipitator is energized at precipitating voltages from a full wave rectifier 12, to which it is conductively connected through a decoupling impedance, shown as inductor 13, which permits the development thereacross of incremental voltages applied pulsewise for sampling the response to the precipitator to higher voltages. The rectifier 12 is returned to ground and electrode 11 through series resistor .14 which operates to supply a signal to a spark detector, as will be further described.

Rectifier bridge 12 is energized by the main step-up transformer 15 whose primary is energized through a surge limiting resistor 16 from a variable autotransformer 17, provided with a motor-driven slider 18, to vary its output potential and the applied operating potential normally energizing the precipitator.

The applied operating potential energizing the precipitator at precipitating voltages is controlled by a reversible motor 20 mechanically connected to slider 18, as will be more apparent below.

The operation of motor 20 is sequenced in dependency on the operating conditions of the precipitator. In particular, sensor means are provided comprising spark detector 31 to provide a momentary output voltage during a transient discharge surge through resistor 14 for the operation of a control relay, as well be later described in connection with the operation of the sampling pulse genrator system and the control network. The latter operates in dependency on the development of sparkover surges at particular operational phases with respect to the application of incremental voltage to the normal precipitator energizing potentials.

The sequential phase relationships in the operation of this system are determined by a synchronous timing motor 24 energized from a power supply frequency source 25. Motor 24 recurrently operates, in predetermined timing sequence, a series of control switches.

Motor 24 is camwise connected, by a mechanical drive shown schematically, to switches 27 and 28 connected in series in line 29 to drive the motor 20 when they are simultaneously closed to increase the operating potential on the precipitator. Motor 20 is returned to the AC. source by line 30. Line 29 includes a pair of normally closed relay contacts, so that the closure sequence of switches 27 and 28, as shown in FIG. 5A, periodically activates motor 20, absent other control signals, to effect an incremental adjustment increasing the operational potential on the precipitator. The magnitude of voltage adjustment thus applied will be selected at a desired value for which the various mechanical couplings between motor 24 and switches 27 and 28, and motor 20 and slider 18 of the autotransformer, will be appropriately designed. Obviously, again absent other control signals, on the subsequent cycle of operation of the timing motor 24, the applied operating potential to the precipitator will again be adjusted for another increase in operating potential. The frequency at which these incremental adjustments are developed can be selected as desired to occur a few seconds apart in most instances. For some applications, it may be necessary to adjust the precipitator potential in response to more rapidly changing values of sparkover potential, in which case the cycling rate established by motor 24 would be increased.

The operation of the system as so far described would tend to establish an aggregate increase in operating potential on the precipitator such as to drive the latter in substantially continuous sparkover. This is prevented, and the operating potential established at a level below sparkover, by the application of sampling potentials incrementally applied to the operating potential developed by rectifier bridge 12. Voltage pulse means are operated in appropriate phase by motor 24 to develop the incremental voltages. For this purpose, motor 24 is camwise connected through a mechanical drive to switch 32 which is closed over a time phase interval synchronized with switch 27, as shown in FIG. 5. Switch 32 is connected via a series condenser 33, in parallel with resistor 34, to the energizing coil of relay 35. Since condenser 33 is normally discharged by resistor 34, the coil of relay 35 is momentarily energized to operate its contacts during the charging period of condenser 33. The closure period is shown in FIG. 5.

During the energization of relay 35, its contacts 35A momentarily connect voltage source 36 to amplifier 37 and thus generate an output voltage pulse for incremental application to the precipitator operating potential. For this purpose, amplifier 37 feeds a pulse unblocking amplifier output tube 38 through driver amplifier 39. The output of the power amplifier 38 is delivered through a pulse transformer 40, whose secondary is coupled to precipitator electrode 10 through a direct potential blocking condenser 41. Consequently, an output pulse operating to incrementally increase the operating potential applied to the precipitator from rectifier bridge 12 is momentarily developed on the precipitator. The applied pulse voltage is developed across decoupling impedance 13 so that the voltage pulse means is not loaded by the power supply network, including rectifier bridge 12.

Thus, timing motor 24 energizes relay 35 for a brief period after closure of switch 32, which in turn by contacts 35A applies an incremental voltage to the precipitator in the phase position shown opposite relay 35 in FIG. 5B. As the operating potential applied to the precipitator from rectifier bridge 12 is raised stepwise on each cycle of timing motor 24, eventually the combined voltage supplied by rectifier bridge 12 and the voltage pulse means under opeartion of contacts 35A together will exceed the instantaneous sparkover potential. The voltage relations under discussion are shown in FIG. 2, line 45 indicates the operating potential, 46 indicates the instantaneous sparkover potential, and pulse voltage 47, added to the precipitator operating potential, exceeds the sparkover potential. Under these conditions, a sparkover will occur during the incremental voltage pulse and spark detector 31 will supply potential voltage to the coil of relay 50 to cause contacts 50A to close. Regardless of the phase of the sparkover surge during energization of relay 35, the current path energized by contacts 50A will necessarily occur while contacts 35B of relay 35 are closed, to energize relay 55. Relay through contacts 55A then will energize relay 60. As shown in FIG. 5, relay includes time delay means for holding the relay in an activated condition in response to a short energizing signal. Relay 60 includes normally closed contacts 60A in line 29, in series with switches 27 and 28 which are simultaneously operated into closed series position on each cycle under sequencing by timing motor 24. Consequently, line 29, in the event that pulse 47 attains or exceeds the instantaneous sparkover potential of the precipitator, is prevented from energizing control motor 20 and the existing precipitator potential 45 is preserved. It will therefore be understood that the present invention stabilizes the operating potential of the precipitator at an energizing voltage having a predetermined voltage differential below the instantaneous sparkover potential. The system so operates under control of timing motor 24, voltage pulse generator means controlled by relay contacts 35A, and spark detector 31 which detects percipitator response to the incremental voltage.

In FIG. 2, the output voltage coupled through pulse transformer 40 as a sampling pulse applied to increase the voltage across the precipitator electrodes, is shown as a unipotential pulse. For supplying such a pulse, voltage source 36 would comprise a source of constant potential of appropriate polarity to gate output tube 38 into conduction. For reasons of efficiency, however, in order to minimize loss of precipitator energy and voltage during sparkover, it is preferred to apply to the precipitator electrodes a voltage pulse comprising a high frequency wave train of a few cycles, such as five to ten, or greater, of high frequency voltage. The frequency of the applied incremental pulse would preferably lie from 50 kilocycles to 100 kilocycles or higher.

The application of a unipotential incremental pulse voltage to the precipitator to effect a sparkover necessarily increases the precipitator potential and additionally tends to maintain heavy ionization until the end of the applied pulse. Consequently, a relatively high density of ionization may be effected which tends additionally to extend the recovery time and proportionately to discharge the electrostatic energy stored in the precipitator. On the other hand, the application of a short high frequency wave train to the precipitator elements alternately raises and lowers instantaneous potential across the precipitator elements during the applied pulse so as to result in no increase in the average applied potential. At the same time, on the additive half-cycles of the applied energy, precipitator conditions are sampled in the voltage regime above the operating potential, and as these voltages attain sparkover potential, sparkover will result. There is no tendency, however, for the applied alternating potential to sustain the sparkover condition until the end of the pulse, nor is the average potential raised and held at the higher value during the pulse, and in fact, deionization is promoted by the rapid voltage decline effected by the applied alternating voltage below the existing operating potential supplied by rectifier bridge 12, immediately following each crest of the aggregate voltage developed by the voltage pulse means and the power energizing means during the sampling pulse.

Consequently, it is preferred that voltage generator 36 should comprise a high frequency oscillator operating at a predetermined amplitude for supplying, during a momentary closure of contacts 35A, a high frequency voltage wave train. Such a wave train is shown at 65 in FIG. 4 developed incrementally with respect to operating potential 66 supplied to the precipitator by rectifier bridge 12 and, in this instance, shown as exceeding the instantaneous sparkover potential 67 of the precipitator.

The present invention contemplates further provision for lowering the energizing potential applied from rectifier bridge 12 to the precipitator in the event that the instantaneous sparkover potential should drop so as to tend to produce sparkover at the operating potential. Such sparkover would produce high frequency output from spark detector 31, and it is known in the art to supply means for reducing precipitator potential in response to increased spark rate. Obviously, however, it is inefiicient to permit an electrostatic precipitator to operate at a high spark rate with its attendant energy loss and inefficiency of collection. Such conditions are precluded, according to the present invention, by applying to the precipitator between each of the sampling pulses developed from voltage source 36, an intervening sampling pulse of lesser magnitude. If sparkover occurs during any of such smaller amplitude incremental pulses, a phenomenon necessarily occurring prior to operation in a regime where sparkover occurs at the operating potential supplied by rectifier bridge 12, control motor is reversely operated to decrease the alternating voltage applied through transformer 15 to the precipitator energizing network. The operation employing two alternate sets of sampling pulses is shown in FIG. 3, where previously described pulses 47 are developed incrementally with respect to the output of rectifier 12 shown at 45, at a potential below the instantaneous sparkover voltage 46. Under these conditions, timing motor 24 has raised potential 45 and potential 47 together to exceed sparkover potential. Under these conditions, contacts 60A inhibit further increase of the precipitator operating potential under the sequence established by timing motor 24. As sparkover potential 46 transiently falls, this potential level will of necessity first encounter sparkover conditions during subsistence of the lower voltage sequence of sampling pulses 68. When this occurs, timing motor 20 is operated to reduce potential 45 applied to the precipitator before conditions permitting sparkover at potential 45 occur. Thus, the overall spark rate is controlled in this system so that at no time is excessive spark rate with its attendant inefficiencies permitted.

For the purpose of lowering the energizing potential supplied to the rectifier bridge 12, a second voltage source 75 is established for making available sampling pulses of substantially lower potential than those shown at 47 in FIGS. 2 and 3. To control the generation of these pulses, timing motor 24 is camwise connected to switch 76 which closes during the phase ranges shown in FIG. 5. Again, switch 76 is connected through capacitor 77 and resistor 78 to momentarily energize relay 80 during the phase interval shown in FIG. 5 from common bus 81 normally supplied with direct current potential by rectifier bridge 12. Relay 80 then operates to close normally open contacts 80B. In the event the system is in the condition shown in FIG. 3, no sparkover will occur on the development of the pulse 68, relay contacts 50A remain open, and the applied voltage from rectifier bridge 12 remains stabilized. If however, a sparkover is developed in response to the closure of contacts 80A feeding a momentary voltage pulse through condenser 41 to the precipitator, contacts 50A will close during the closure of contacts 80B. This series circuit thus energizes relay 85 to close contacts 85A and energize relay whose contacts remain in an actuated position for a prolonged timing period after energization, extending into coincidence and overlapping the time delay actuation of the contacts of relay 60, as shown in FIG. 5C. Where sparkover is produced by pulse 68, it will be understood that the succeeding higher amplitude pulse 47 necessarily produces sparkover. Thus, under these conditions, lead becomes operative through the simultaneous coincident closure of contacts 60B and QOB of relays 60 and 90 to energize control motor 20 to decrease the voltage supplied to the precipitator from rectifier bridge 12. Consequently, in the event sparkover potential 46 of FIG. 3 declines so that sparkover occurs during the lower amplitude sampling pulse 68, as well as during pulse 47, voltage 45 is lowered to preclude operation of the precipitator in a regime producing a high sparkover rate as a result of the application to the precipitator of the output potential from bridge 12. Manifestly, by selection of the absolute and relative magnitudes of pulses 68 and 47, the average precipitator voltage may be maintained at the desired predetermined value below the instantaneous sparkover potential.

As in connection with voltage source 36, it is preferred that voltage source 75 supply pulses of a series of cycles of high frequency voltage 69 on closure of contacts 80A for the purposes of minimizing energy loss on sparkover. Oscillator circuits for producing such voltages at predetermined amplitudes are conventional in the art and need not be described in detail. As described above, the preferred range of high frequency energy lies above about 50 kilocycles. It might be presumed that the upper limit of the pulsed energy frequency might be practically established with respect to the very substantial lumped capacity of an industrial precipitator system as a controlling parameter. In this connection, it should be noted, however, that the physical construction of many precipitators would be seen at high frequency as open-ended radio frequency transmission lines. The impedance of such structures at these high frequencies are not such as would be calculated on the basis of the aggregate lumped capacitance. Consequently, a relatively wide rang of voltage pulse wave train frequencies is available in engineering the application of the present invention to precipitator structures.

What is claimed is: 1. An electrostatic precipitator system comprising: a source of direct current potential, a precipitator comprising a pair of electrodes, means conductively connecting the electrodes with the source for continuous energization at precipitating voltage, voltage pulse means electrically connected to the precipitator electrodes recurrently operative to develop incremental voltage for application to the precipitator electrodes, sensor means for detecting precipitator response to the incremental voltage, and control means for the source responsive to the sensor means to increase the direct current potential on absence of precipitator sparkover during development of the incremental voltage, whereby the precipitator normally operates under source energization at precipitating voltage below sparkover potential and reaches sparkover potential under combined energization from the source and the voltage pulse means. 2. The precipitator system of claim 1 wherein: the voltage pulse means comprises means to develop high frequency incremental voltage. 3. An electrostatic precipitator system comprising: a source of direct current potential, a precipitator comprising a pair of electrodes, means conductively connecting the electrodes with the source for continuous energization at precipitating voltage comprising inductor means, voltage pulse means electrically connected to the inductor means recurrently operative to develop incremental voltage across the inductor means for application to the precipitator electrodes, sensor means for detecting precipitator response to the incremental voltage, and control means for the source responsive to the sensor means to increase the direct current potential on absence of precipitator sparkover during development of incremental voltage and to decrease the direct current potential when the direct current potential exceeds a predetermined value below sparkover potential. 4. An electrostatic precipitator system comprising: a source of direct current potential, a precipitator comprising a pair of electrodes, means conductively connecting the electrodes with the source for continuous energization at precipitating voltage, voltage pulse means electrically connected to the inductor means recurrently operative to develop incremental voltages successively at two different values at the precipitator electrodes, sensor means for detecting precipitator response to the incremental voltages, and control means for the source responsive to the sensor means to increase the direct current potential on absence of precipitator sparkover during development of the higher incremental voltage and to decrease the direct current potential on precipitator sparkover during development of the lower incremental voltage. 5. The precipitator system of claim 4 wherein: the control means is operative to maintain constant direct current potential on presence of precipitator sparkover during development of the higher incremental voltage followed by absence of precipitator sparkover during development of the lower incremental voltage.

6. The precipitator system of claim 4 wherein:

the voltage pulse means comprises means to develop high frequency incremental voltage.

7. The precipitator system of claim 6 wherein:

the control means comprises servo-motor means reversely actuatable in dependency on the sensor means.

8. The precipitator system of claim 7 further including:

timing motor means coupled to the voltage pulse means to sequence its operation.

9. The precipitator system of claim 8 wherein:

the voltage pulse means comprises a pair of alternately operated high frequency generators the amplitudes of the output voltages of which differ.

10. An electrostatic precipitator system comprising:

a source of adjustable direct current potential,

a precipitator comprising a pair of electrodes,

means conductively connecting the electrodes with the source for continuous energization at precipitating voltage,

voltage pulse means electrically connected to the precipitator electrodes recurrently operative to develop incremental voltages successively at two different values at the precipitator electrodes,

sensor means for detecting precipitator response to the incremental voltages, and

control means for the source responsive to the sensor means operative to maintain constant direct current potential on presence of precipitator sparkover during development of the higher incremental voltage followed by absence of precipitator sparkover during development of the lower incremental voltage.

11. A control system for a direct current energized electrostatic precipitator comprising:

voltage pulse means recurrently operative to develop an output voltage pulse of fractional amplitude relative to precipitation voltage,

coupling means fed by the pulse means connectible to a precipitator system to increase the operating voltage during pulses,

sparkover sensor means connectible to a precipitator system to supply an output signal on sparkover,

servo-motor means connectible with a main power supply for a precipitator system, and

control means for the servo-motor means operative to actuate the same on absence of sensor output signal during operation of the voltage pulse means.

12. The control system of claim 11 wherein:

the voltage pulse means comprises means to alternate the output pulse amplitude between two voltage values, and

the control means comprises means to reversely actuate the servo-motor means on presence of sensor output signal during operation of the voltage pulse means to develop pulses of the lower of the two voltage values.

13. An electrostatic precipitator system comprising:

an adjustable source of potential for energizing the precipitator at precipitating voltage,

a precipitator comprising a pair of electrodes,

means coupling the source to the electrodes,

voltage pulse means recurrently operative to develop high frequency alternating voltage pulses incrementally applied across the precipitator electrodes,

sensor means for detecting precipitator sparkover during application of the incrementally applied high frequency voltage, and

control means for the source operative in dependency on the sensor means to adjust the precipitating voltage to a value slightly below the instantaneous sparkover potential.

(References on following page) References Cited UNITED STATES PATENTS Heinrich 55123 Lissman 552 Wintermute 55-2 Hahn 55139 Heinrich et a1. 55--2 Newman 55105 White 55139 Hall 317157 X Backer et a1 55105 Willison 55-105 X Hall 55105 Hall et a1. 55--105 Wasserman 55105 Berg 55-105 10 3,039,252 6/1962 Guldemond 6t a1. s5 105 3,039,253 6/1962 Van Hoesen et a1. 55105 FOREIGN PATENTS 248,429 10/1963 Australia. 5 657,376 3/1938 Germany.

1,326,143 3/1963 France.

OTHER REFERENCES HARRY B. THORNTON, Primary Examiner.

D. E. TALBERT, JR., Assistant Examiner.

U.S. Cl. X.R. 

